Oxide wire bond insulation in semiconductor assemblies

ABSTRACT

A semiconductor integrated circuit package is provided with insulated bonding wires. The semiconductor die is mounted to a base of either a leadframe or a grid-array package. A plurality of bonding wires are bonded between bonding pads on the semiconductor die and bonding fingers at inner ends of package conductors. The bonding wires have an oxygen-plasma oxide formed thereupon to thereby provide electrically-insulated bonding wires to prevent short-circuits between adjacent bonding wires. After wire bonding of the bonding wires, the bonding wires are subjected to an oxygen plasma to form an insulating oxide on the bonding wires to prevent short-circuits with adjacent wires. The wires are aluminum or copper with an oxygen-plasma oxide formed thereupon. An oxygen-plasma oxide is simultaneously formed on a leadframe and is removed from the outer ends of the leads by blasting with a medium.

This application is a division of Ser. No. 08/997,295 filed Dec. 23,1997.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to packaging of semiconductor devices and, moreparticularly, to techniques for insulating bonding wires during assemblyof an integrated-circuit package.

2. Prior Art

The trend in integrated circuit packaging is to have the spacing betweenbonding pads on an integrated circuit die become smaller so that thepitch of the bonding pads gets tighter. For this reason and for otherreasons, such as this use of multiple bonding tiers, the length of thebonding wires in IC packages are getting longer and longer bonding wireshave a tendency to electrically short-circuit together.

Various attempts have been made to electrically insulate bonding wiresto avoid having adjacent wires touching each other and electricallyshort-circuit. One technique provides insulation on the wires prior towire bonding. Insulating the bonding wires prior to bonding createdproblems such as modification of wire-bonding machines, increased costs,decreased yields, and charring of the insulation material whichcontaminates the integrated-circuit die.

Another technique provides for coating the bonding wires with a siliconespray after wire bonding. Using this so-called "wet" technique, thesprayed silicone material can get on the leadframe or substrate on whichthe die is mounted. During encapsulation of the die and bonding wires,this silicone material causes a loss of adhesion of the encapsulatingmaterial to the leadframe or substrate. This silicone spray alsoprevents good adhesion of plating materials on the leadframe.

Consequently, a need exists for a dry technique for electricallyinsulating bonding wires to prevent electrical short circuitstherebetween.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a technique forproviding electrical insulation to the bonding wires of anintegrated-circuit package during assembly of the package to preventelectrical short circuits between adjacent bonding wires in theintegrated-circuit package.

In accordance with these and other objects of the invention, asemiconductor integrated-circuit die is singulated and die-attached tothe leadframe or a substrate. Wire bonds are formed between the die andthe package leads using aluminum or copper bonding wires. Next, thewire-bonded ICs are subjected to an oxygen plasma process whereenergetic oxygen ions bombard the aluminum or copper bonding wires andthe leads to produce an insulating layer of aluminum oxide or copperoxide on the bonding wires. The assembly is then encapsulated as eithera molded plastic package, a grid-array package including a glob-toppackage, or a pin-grid array package. For a molded plastic package witha leadframe, the oxide is then removed from the leads of a moldedplastic package by a media-blasting process and the leads are platedwith Pb/Sn coating.

A method is provided according to the invention for packaging asemiconductor integrated circuit with insulated bonding wires. Themethod includes: attaching a semiconductor die to a mounting base andwire-bonding a plurality of aluminum or copper bonding wires betweenrespective bonding pads on the semiconductor die and respective bondingfingers at the inner ends of leads of a leadframe. After wire bonding,the bonding wires are subjected to an oxygen plasma to form an aluminumoxide on the plurality of bonding wires to thereby provideelectrically-insulated bonding wires to prevent short-circuits betweenadjacent bonding wires of the package. For a package with a leadframe,the leads are also subjected to the oxygen plasma to form an oxide onthe leads. The semiconductor die, the bonding wires, and the bondingfingers are encapsulated to expose the oxidized outer ends of the leads.The oxide on the exposed outer ends of the leads of the leadframe areremoved by blasting with a media. For a grid-array package, such as aball-grid array package or a pin-grid array package, a die is mounted toa polymer substrate and the bonding wires are attached to conductorsformed on the polymer substrate. The bonding wires are then subjected toan oxygen plasma to provide an insulating oxide thereupon.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 is a sectional view showing a bonding wire connected between abonding pad on an integrated-circuit die and a bonding finger of aleadframe of a molded plastic package assembly.

FIG. 2 is a sectional view showing apparatus using an oxygen plasma forgrowing an Al₂ O₃ aluminum oxide layer on bonding wires of the moldedplastic package assembly.

FIG. 3 is a sectional view showing formation of the Al₂ O₃ aluminumoxide layer on the aluminum bonding wire and bonding pad and alsoshowing formation of an oxide layer on the bonding finger of theleadframe of the molded plastic package assembly.

FIG. 4 is a sectional view showing encapsulation of the semiconductordie, the bonding wires, and the inner end of the bonding finger toexpose the oxidized outer end of the lead of the molded plastic packageassembly.

FIG. 5 is a sectional view showing apparatus for removal of the oxide onthe exposed outer end of the lead of the leadframe of the molded plasticpackage assembly by blasting with a media.

FIG. 6 is a sectional view showing the step of plating the exposed outerend of the lead of the leadframe of the molded plastic package assemblywith a Pb/Sn coating.

FIG. 7 is a sectional view showing the step of forming the outer end ofthe lead of the molded plastic package assembly.

FIG. 8 is a sectional view showing a bonding wire connected between abonding pad on an integrated-circuit die and a copper trace of agrid-array package.

FIG. 9 is a sectional view showing apparatus using an oxygen plasma forgrowing an insulating layer on bonding wires of the grid-array package.

FIG. 10 is a sectional view showing formation of an insulating layer onthe bonding wire of the grid-array package.

FIG. 11 is a sectional view showing encapsulation of the semiconductordie, the bonding wires, and the inner end of the copper trace to exposethe outer end of the lead grid-array package.

FIG. 12 is a sectional view showing attachment of solder balls to thegrid-array package.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims.

FIG. 1 shows a structure which includes singulated semiconductorintegrated-circuit die 10 mounted to a suitable mounting base 12 such asa die-attach pad or a separate substrate with a layer 14 of anappropriate bonding material, such as a silver-filled epoxy material.The substrate 12 is a die-attach pad of a leadframe or, optionally, aseparate thermally conductive substrate. A bonding wire 16 has one endultrasonically bonded to a bonding pad 18 on the top surface of theintegrated-circuit die 10. The top surface of the die 10 is covered witha protective oxide layer 20 having openings provided over the bondingpad 18. The other end of the bonding wire is ultrasonically bonded to abonding finger 22 at the inner end of a lead 24 of the leadframe. Thebonding wire 16 is typically made of a conductive material such asaluminum or copper, which can be oxidized to form an insulating layer onthe surface of the bonding wire. The bonding pad 18 on the die istypically made of aluminum and the leadframe is typically made of copperor silver-plated copper.

FIG. 2 illustrates an oxygen-plasma process for forming an insulatinglayer on the structure of FIG. 1. The dotted arrows represent energeticO- ions in an oxygen plasma surrounding the structure.

FIG. 3 illustrates the result of subjecting the structure of FIG. 1 tothe oxygen plasma environment of FIG. 2. The energetic O- ions in theplasma react at the surface of the aluminum or copper bonding wire 16 toform an insulating coating 25 of aluminum oxide or copper oxide on thebonding wire 16. The insulating coating 25 on each of a plurality ofbonding wires in a package provide electrical insulation betweenadjacent bonding wires in order to prevent electrical short-circuits, orshorts, therebetween.

The energetic O- ions in the plasma also react at the surface of thecopper or silver-coated leadframe to form an insulating coating 26 ofcopper oxide or silver oxide on the leads and die-attach pad of theleadframe. An oxide coating is also formed on the aluminum bonding pad18.

FIG. 4 illustrates a molded plastic package body 28 formed of a plasticmolding compound material. The molded package body 28 encapsulates thesemiconductor die 10, the insulated bonding wire 16, and the inner endof the bonding finger 22. The oxide layer 26 at the outer ends of thelead 24 is exposed after this encapsulation.

FIG. 5 illustrates apparatus for removal of the oxide layer 26 on theexposed outer end of the lead 20 of the leadframe shown in FIG. 4. Aseries of nozzles, typically shown as 30, are used to direct a highpower jet using a media such as a mixture of water and glass beads,typically shown as 32, for blasting away the oxide layer 26 at the outerends of the lead 24.

FIG. 6 illustrates the exposed outer end of the lead 24 having a Pb/Sncoating 34 formed thereupon. FIG. 7 illustrates the lead 24 of themolded plastic package being bent to a suitable profile, such as, forexample, a gull wing configuration.

FIG. 8 illustrates a package structure for a grid-array package whichincludes a singulated semiconductor integrated-circuit die 40 mountedand attached to a die-attach pad portion 42 of a conductive copper layerwith a layer 44 of an appropriate bonding material, such as asilver-filled epoxy material. The copper layer extends over an insulatedsubstrate 46 formed, for example, of a polymer material, such as anepoxy or polyimide material. The copper layer includes a plurality ofperipheral conductive traces, typically shown as 48. A top solder mask50 is formed over the copper trace 48 with openings over the inner endsthereof. A bottom solder mask 52 extends over the lower surface of theepoxy substrate 46 with openings formed therein through which extendgold-plated conductive pads 54, 55 for connection to solder balls in aball-grid array package. Grid-array packages also include pin-grid arraypackages where the pads 54, 55 are replaced with projecting pins.

A typically-illustrated bonding wire 56 has one end ultrasonicallybonded to a bonding pad 58 formed on the top surface of theintegrated-circuit die 40. The other end of the bonding wire 56 isultrasonically bonded to a bonding area 60 at the inner end of theconductor area 48. The bonding wire 56 is typically made of a conductivematerial, such as aluminum or copper, which can be oxidized to form aninsulating layer on the surface of the conductor. The bonding pad 58 istypically made of aluminum which is oxidized to form an insulating layerthereupon.

FIG. 9 illustrates an oxygen-plasma process for forming an insulatinglayer on the bonding wire 56 of FIG. 8, where the dotted arrowsrepresent energetic O- ions in an oxygen plasma surrounding thestructure for the grid-array package.

FIG. 10 illustrates the result of subjecting the structure of FIG. 8 tothe oxygen plasma environment of FIG. 9. The energetic O- ions in theplasma react at the surface of the aluminum or copper bonding wire 56 toform an insulating coating 62 of aluminum oxide or copper oxide on thebonding wire 56 to provide electrical insulation between adjacentbonding wires in order to prevent electrical short-circuits, or shorts,therebetween. The energetic O- ions in the plasma do not react at thegold surface of the gold-plated conductive pads 54, 55, leaving themready to have solder balls attached thereto.

FIG. 11 illustrates a glob-top package body 64 formed of an epoxymaterial to encapsulate the semiconductor die 40, the insulated bondingwire 56, and the inner end of the bonding area 60 at the inner end ofthe conductor area 48. FIG. 12 illustrates attachment of solder balls66, 67 to the conductive pads 54, 55 of the grid-array package. It isintended that the grid-array package, such as a pin-grid array package,are provided with oxidized insulated bonding wires in accordance withthe present invention.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto and theirequivalents.

I claim:
 1. An improved semiconductor integrated circuit package withelectrically-insulated bonding wires, comprising:a semiconductor dieattached to a mounting base; a plurality of aluminum bonding wireshaving respective first ends wire-bonded to respective bonding pads onthe semiconductor die and having respective second ends bonded torespective bonding fingers at inner ends of leads of a leadframe; thealuminum bonding wires having an oxygen-plasma oxide formed thereupon tothereby provide electrically-insulated bonding wires to preventshort-circuits between adjacent bonding wires; and the leads of theleadframe also having an oxygen-plasma oxide formed thereupon except atthe exposed outer ends thereof.
 2. The improved package of claim 1wherein the semiconductor die is attached to a die-attach pad of theleadframe.
 3. The improved package of claim 1 wherein the semiconductordie is attached to a separate substrate.
 4. The improved package ofclaim 1 wherein the exposed outer ends of the leads of the leadframe areplated with a Pb/Sn coating.
 5. The improved package of claim 1including a plastic package body which is molded around thesemiconductor die, the bonding wires, and the bonding fingers.
 6. Animproved semiconductor integrated circuit package withelectrically-insulated bonding wires, comprising:a semiconductor dieattached to a mounting base; a plurality of metal bonding wires havingtheir respective first ends bonded to respective bonding pads on thesemiconductor die and having their respective second ends bonded torespective bonding fingers at inner ends of package conductors; and theelectrically-insulated metal bonding wires having an oxygen-plasma oxideformed thereupon to thereby provide electrically-insulated bonding wiresto prevent short-circuits between adjacent bonding wires.
 7. Theimproved package of claim 6 wherein the mounting base includes adie-attach pad of a leadframe.
 8. The improved package of claim 6wherein the mounting base includes a separate thermally conductive andelectrically insulated substrate.
 9. The improved package of claim 6wherein the bonding wires include aluminum wires with an oxygen-plasmaaluminum oxide formed thereupon.
 10. The improved package of claim 6wherein the bonding wires include copper wires with an oxygen-plasmacopper oxide formed thereupon.
 11. The improved package of claim 6wherein the bonding wires are wire-bonded to the bonding fingers of aleadframe which has an oxygen-plasma oxide formed thereupon except atthe outer ends of the leads.
 12. An improved integrated-circuitgrid-array package, comprising:a semiconductor die mounted to aninsulated grid-array substrate; a plurality of bonding wires bonded attheir first ends to respective bonding pads on the semiconductor die andbonded at their second ends to respective bonding fingers at inner endsof conductive traces formed on the substrate; the bonding fingers havingan oxygen-plasma oxide formed thereupon to thereby provideelectrically-insulated bonding wires to prevent short-circuits betweenadjacent bonding wires; and wherein the semiconductor die, theelectrically-insulated bonding wires, and the bonding fingers areencapsulated with a packaging material.
 13. The improved package ofclaim 12 wherein the package includes gold-plated solder-ball pads onthe substrate of a ball-grid array package.
 14. The improved package ofclaim 12 wherein the substrate is a polymer substrate.